Heat death

Humans, like all mammals, are heat engines; surviving means having to continually cool off, as panting dogs do. For that, the temperature needs to be low enough for the air to act as a kind of refrigerant, drawing heat off the skin so the engine can keep pumping. At seven degrees of warming, that would become impossible for portions of the planet’s equatorial band, and especially the tropics, where humidity adds to the problem. And the effect would be fast: after a few hours, a human body would be cooked to death from both inside and out.

At eleven or twelve degrees Celsius of warming, more than half the world’s population, as distributed today, would die of direct heat. But at just five degrees whole parts of the globe would be literally unsurvivable for humans.